The present invention relates generally to the pulp and paper industry. More specifically, the present invention relates to apparatuses for washing pulp and, more specifically, to apparatuses for washing pulp which include a plurality of discharge conduits. The present invention is directed toward devices for regulating the flow between the discharge conduits to compensate for variations in the operation of the apparatus and in the feed slurry.
In currently used methods of making pulp from wood stock, the wood, which may be in the form of wood chips, is heated in a digester. In the digester, the lignin is chemically dissolved and heated to free the cellulose fibers so they can be reformed into paper. The end product of the digestion process is cooked pulp fibers.
The cooked pulp fibers are then blown into a tank where the steam flashes off. Black liquor is added to the blow tank to dilute the pulp and form a slurry. The pulp slurry is then further diluted and transferred to a pulp washer.
In the pulp washer, a mat or sheet is formed from the slurry and the black liquor is extracted as filtrate. Wash liquor is added to displace the black liquor and is also subsequently extracted. The extracted liquids are recycled to the extent possible and their recycled use depends upon the solids content in the extracted liquid. It is therefore a fairly common practice to xe2x80x9csplitxe2x80x9d the filtrate or extracted liquid into a plurality of discharge conduits, depending upon the solids content in the extracted liquid. For example, in rotating drum filters, it is common to extract the initial black liquor filtrate separately and then split the wash liquor filtrate into xe2x80x9ccloudyxe2x80x9d or xe2x80x9cweakxe2x80x9d and xe2x80x9cclearxe2x80x9d or xe2x80x9cstrongxe2x80x9d streams. Some devices employ a third discharge conduit used to extract a xe2x80x9cvery clearxe2x80x9d stream later in the cycle. In other pulp washers or thickeners, the discharge stream can be split into a number of different discharge streams.
Typically, a movable assembly is provided, referred to as a splitter or splitter valve, that segregates the liquor or filtrate flow into two or more flow streams. Typical prior art splitter valves are either fixed or manually adjustable. As a result, they are installed for a specific set of operating conditions and their performance deteriorates as these conditions change. In most pulp mills, the parameters which affect the most efficient position of the splitter valve can vary constantly and therefore the currently available fixed or manually adjustable splitter valves seldom operate at their optimum setting or position.
Further, when operating a multi-stage washer, increasing the speed of the operation or the speed of rotation of the drum can result in dirty black liquor being carried forward to the subsequent washing stage. As a result, the efficiency of the washing operation is compromised. One condition contributing to the forward movement of black liquor into a second stage or a washing stage is the inappropriate setting of the splitter valve. If the splitter valve were set to permit a sufficient discharge rate of the black liquor, it would not be carried forward into a subsequent washing stage.
Finally, if the splitter valve is not accurately positioned, the concentration of solids in the collected filtrate stream will be adversely affected. Specifically, a xe2x80x9cclearxe2x80x9d or xe2x80x9cweakxe2x80x9d stream could result which has an unacceptably high solids concentration. Further, xe2x80x9ccloudyxe2x80x9d or xe2x80x9cstrongxe2x80x9d streams with unacceptably low solids concentrations could result thereby adversely affecting the system""s efficient use of wash liquor and efficient recycling of the various wash liquor discharge streams.
Accordingly, there is a need for an improved system for controlling the position of the splitter valve in pulp washing apparatuses. Improvements in the control of splitter valves in the system will result in faster operation of the apparatuses as well as improved recycling of filtrate streams.
In satisfaction of the aforenoted needs, the present invention provides an adjustable splitter valve and control system for a pulp washing apparatus that includes a displacement zone for displacing fluid from a pulp mat formed from a pulp slurry and a plurality of discharge conduits for discharging fluid from the displacement zone. The splitter valve and control system includes a movable valve body for directing fluid from the displacement zone to one or more discharge conduits. The valve body is connected to an actuator and the actuator is in communication with a controller. The controller is in communication with at least one sensor disposed in each discharge conduit. The sensors measure at least one physical property of the fluid that flows through each discharge conduit when the valve body is in a position so as to direct fluid through that conduit. Each sensor sends a signal indicating a measured value for the physical property of the fluid flowing through each discharge conduit to the controller. The controller then compares the measured value to a predetermined value range for the respective discharge conduit. In the event the measured value falls outside of a predetermined value range for the conduit or one or more conduits, the controller sends a signal to the actuator to move the valve body to a more appropriate position.
In an embodiment, the sensors measure the conductivity of the fluid flowing through the discharge conduits.
In an embodiment, the sensors measure the solids concentration of the fluid flowing through the discharge conduits.
In an embodiment, the plurality of discharge conduits includes a first conduit for primarily discharging black liquor from the washing apparatus, the black liquor having a high solids concentration and high conductivity. The discharge conduits also include a last conduit for discharging weak, or substantially clean wash liquor from the washing apparatus. The weak, or very clean wash liquor, has a low solids concentration and a low conductivity. The plurality of discharge conduits also include one or more discharge conduits spaced between the first discharge conduit and the last discharge conduit. These additional discharge conduits discharge fluid having a progressively decreased dissolved solids content and a decreased conductivity as the conduits are spaced closer to the last discharge conduit. In other words, the discharge conduit disposed immediately adjacent to the first discharge conduit discharges fluid having a relatively high dissolved solids content and high conductivity in comparison to the fluid discharged by the discharge conduit disposed immediately adjacent to the last discharge conduit.
In an embodiment, the present invention provides a method for controlling the position of a filtrate splitting device in a pulp washer that includes a plurality of discharge conduits as described above, each conduit including a sensor that is in communication with a controller, the controller being in communication with an actuator that translates signals from the controller to a repositioning movement of the splitter valve. The method includes the steps of measuring a physical property of the fluid being discharged from one or more of the discharge conduits, transmitting a signal reflective of the measured value to the controller, comparing the measured value with a predetermined operating range and, in the event the measured value falls out of the predetermined optimal range, sending a signal to the actuator to thereafter reposition the splitter valve.
In an embodiment, the sensors measure the conductivity of the fluid being discharged through each conduit and send a signal reflective of the measured conductivity to the controller. The controller then compares the measured conductivity value with a preferred conductivity range for each discharge conduit. If the measured value in the first conduit is less than the preferred range, the controller sends a signal to the actuator to move the splitter valve towards the first discharge conduit so that less of the lower conductivity (low solids concentration) fluid is discharged to the first conduit. In the event the measured conductivity value in the first conduit is higher than the preferred conductivity range for the respective discharge conduit, the controller sends a signal to the actuator to move the splitter valve towards the second discharge conduit to thereby reduce the amount of the low conductivity (low solids concentration) fluid being discharged to the first conduit from the pulp washer and to thereby minimize the rate at which wash liquor is consumed. In both cases, the splitting of high and low solids concentration liquors is optimized.
In an embodiment, as the splitter valve moves from the first discharge (black liquor) conduit towards the last (very weak) discharge conduit, the percentage of weak liquor or filtrate that is removed from the pulp washer decreases. As a result, a smaller quantity of wash liquor is utilized in the pulp washer.
It is therefore an advantage of the present invention to provide an improved method of controlling the position of a splitter valve for a pulp washer.
Another advantage of the present invention is that it reduces the use of clean wash liquor in pulp washers.
Another advantage of the present invention is that it more efficiently splits the filtrates and liquors discharged from a pulp washer for a more efficient recycling of these materials.
Yet another advantage of the present invention is that it enables pulp washers to be operated at a faster rate.
Still another advantage of the present invention is that it enables pulp washers to be operated at higher efficiencies.
These and other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings.